1. Field of the Invention
This invention relates to the composition of the material of shell investment molds for casting and directionally solidifying nickel-base superalloys and high-temperature nickel-base eutectic materials which require the mold to be exposed to elevated temperatures greater than from 1680.degree. C. to 1700.degree. C. for extended periods of time of up to 24 hours.
2. Description of the Prior Art
Shell investment molds are employed to produce castings of a wide variety of alloys with a refractory material, compatible with the alloy being cast, forming the inner mold wall. The secondary or back-up coats usually are composed of a high alumina-silicate refractory of an appropriate grain size to insure production of a useable mold. Long periods of time, up to 10 hours, coupled with high temperatures (1500.degree. - 1600.degree. C.) caused undesirable mold defects, such as total collapse after casting the alloys, premature cracking and mold warpage. All of these, of course, produce undesirable and unacceptable castings. Mold-metal reactions, such as "pock marks," were also noted, suggesting that the inner or primary coats became contaminated with excessive amounts of SiO.sub.2, Na.sub.2 O, and other fluxing agents which were not compatible with the superalloy being cast.
In a copending patent application, "Fabrication of Shell Investment Molds," Ser. No. 591,168, and assigned to the same assignee as this patent application, Paul Svec teaches a material composition of at least two different flour grain sizes of fused aluminum and a silica binder. Continued efforts have been directed to develop material compositions which are as good as, or better, than those taught by Mr. Svec and the prior art, particularly a material having more resistance to slumping at elevated temperatures in excess of about 1680.degree. to 1700.degree. C.
An object of this invention is to provide a new and improved process for making shell investment molds for the casting of superalloys which overcomes the deficiencies of the prior art.
Another object of this invention is to provide a new and improved process for making shell investment molds suitable for use in high-temperature mold applications (1680.degree. - 1700.degree. C.).
Another object of this invention is to provide a new and improved process for making shell investment molds having excellent resistance to slumping at elevated temperatures in excess of about 1700.degree. C.
A further object of this invention is to provide a new and improved process for making an improved alumina slurry which, upon firing, will withstand the effects of mold-metal reactions at the mold-metal interface, such as required for the directional solidification of nickel-base superalloys and high-temperatures nickel-base eutectic alloys requiring long solidification periods to obtain the desired cast structure.
Other objects of this invention will, in part, be obvious and will, in part, appear hereinafter.